Conventional plano-stereoscopic display systems produce the depth sense, stereopsis, by presenting appropriate left and right planar images to each respective eye of an observer. For the observer to be able to fuse these two planar images into a single stereoscopic view, the image for one eye must be isolated from the other. If the left eye, for example, also sees all or a portion of the intensity of the right image, there will be a perceived doubling of the image or "ghosting."
In the stereoscopic display system described in U.S. patent application Ser. No. 267,699 (filed Nov. 2, 1988, by L. Lipton, et al.), a field-sequential stereoscopic image is displayed by a monitor. The displayed image is viewed through a pair of liquid crystal shutter assemblies mounted as lenses in an eyeglass frame. By driving the two liquid crystal shutter assemblies 180 degrees out of phase, consecutive image fields are transmitted alternately to each eye. An infrared (IR) transmitter is mounted at the monitor, and an IR receiver is mounted in the frame of the eyewear for receiving an IR synchronization signal broadcast (or transmitted in another manner, such as by radio) from the transmitter. The synchronization signal is supplied from the receiver to a drive circuit for use in generating synchronized drive signals for the liquid crystal shutter assemblies, to switch the liquid crystal shutter assemblies in synchronization with the field rate of the displayed field-sequential image.
With appropriate carrier-less drive signals, very little power is required for driving the liquid crystal shutter elements, so that the drive circuit (and a power supply for the liquid crystal shutter elements and IR receiver) may be embodied in a small battery incorporated within the frame of the eyewear. The power reduction resulting from use of carrier-less driving signals enables use of small, light-weight batteries and allows a user to run the eyewear for a long period of time without replacement or recharging of batteries.
Conventional liquid crystal cells, which may be included in the described stereoscopic system, include a layer of liquid crystal material sandwiched between two flat and parallel glass sheets, coated with substrates on their inside facing surfaces. These substrates are thin, transparent, electrically conductive layers such as indium tin oxide. It is through this layer that an electric field is applied to the liquid crystal material. Another thin coating called an alignment layer is deposited on top of the conductive layer. The alignment layer imposes a preferred orientation on the liquid crystal molecules. Such an orientation is necessary for the shutter to exhibit the desired electro-optic effect. One way in which this orientation effect can be accomplished is to rub the alignment layer with a special material. The rubbing direction on one substrate is parallel (or antiparallel) to the rubbing direction on the other substrate, as taught by Fergason in U.S. Pat. No. 4,385,806.
Such a surface mode liquid crystal cell is a capacitor and can be charged to a high or low electric potential. A surface mode cell may be switched at high speed because only a thin layer of liquid crystal molecules near the substrate actually moves as the electric potential is switched.
The liquid crystal material is in a retardation state when at a low electrical potential, and is in an isotropic state when at a high electrical potential. In the low potential state, the molecules near the surface maintain the alignment imposed on them by rubbing of the director alignment layer, and when in the high potential state, the molecules become aligned parallel to the electric field and are therefore isotropic rather than anisotropic. A typical high potential state is between 25 to 50 volts peak to peak, and a typical low potential state is between 0 to 10 volts peak to peak. By using different voltage settings for the low potential, one can tune the retardation of the liquid crystal cell. Generally speaking, the high voltage state determines how quickly and completely the cell will "turn on," and the low potential state determines the value of retardation. By adjusting the voltages it is possible to vary the dynamic range of the shutter continuously.